Ranger REG said:
Last I heard about "teenaged" Illyana, she was freed from some kind of demonic entity (the source of her powers) and reverted to her much younger age.
As far as I know...
1. Illyana is Piotr Rasputin's kid sister, living with his family in Russia.
2. She comes over to visit, and for some reason stays.
3. The X-men find this weird vaguely cthulhoid island fortress and investigate it. Ilyana is with them.
4. It turns out the fortress is linked to another dimension, called Limbo, ruled by a sorcerer named Belasco. The X-men are trapped by him, but escape.
5. Right after escaping, Belasco tries to kidnap Illyana, Peter grabs a hold of her through the portal in question, but loses touch for a second... and since time works differently in Limbo, she returns as a teen, with awakened mutant powers as well as being a somewhat powerful sorceress (not sure if she had defeated Belasco at this point and assumed rulership over Limbo, or if that came later).
6. It turns out that Illyana's magical powers are tied to her dark side, often called Darkchylde.
7. Years later, Inferno starts as a result of a revolt in Limbo. As part of the resolution of Inferno, Illyana regresses in age to where she should be, or maybe to infanthood. I don't think much is heard from her after that.